1 New advances in geological and geophysical research on the Appalachian orogen.

We invite contributions that discuss new research on the Appalachian orogen. Submissions should include, but are not limited to, geological or geophysical studies or works comprising a combination of both methods.

Sunday, 22 March 2026
8:00 AM - 12:00 PM
CCC, Room 22/23

The Peridotites of Coastal Maine and their Implications for Tectonic Models of the northern Appalachian Orogen

Paper number: 1-7
Program: Technical Sessions
Time: 10:00 AM – 10:20 AM

Authors

Abstract

The Deer Isle (DI) and Little Deer Isle (LDI) peridotites, located in Penobscot Bay, southeastern Maine, provide an ideal case study to explore the geologic context and implications for the regional tectonics of coastal Maine. Based on field relationships and regional context, DI was viewed as mantle, exhumed when Ganderia departed from Amazonia during the Cambrian prior to subsequent incorporation into a subduction complex. We present new data on the trace-element composition from pyroxene, which provides a more reliable indicator for tectonic setting, reveals further complexities not captured through whole-rock analyses, and can better track melt extraction history and/or subsequent melt reactions.

The dominant lithology at DI is harzburgite, with elongated orthopyroxene (En90.1-91.6, Wo0.12-0.17, and Mg#90.6-91.7). The opx display spoon-shaped depleted profiles (< 1x DMM), with a positive HREE slope and LREE enrichments. In one sample, analyzed “cpx,” interpreted as pigeonite, display highly enriched (La/LuN = 1.8-2.0x CI chondrite) negative REE profiles. The dominant lithology at LDI is wehrlite, which is likely a melt reaction product. Two cpx populations are present: low-TiO2 (Mg#91.8, En46.9, Wo48.9) and high-TiO2 (Mg#85.6, En45.1, Wo47.3). One set of LDI cpx exhibits a negative HREE trend and a pronounced MREE hump, reflective of a garnet heritage, and an average La/LuN value of 0.2. Another set of LDI cpx exhibit a “Z” structure, i.e., a positive HREE slope and flat LREE slope, a positive Eu-anomaly, and an average La/LuN value of 0.4. Other elements suggestive of a subduction setting, e.g., the fluid-mobile elements, are enriched (B: 11-1000x and Li: 2-11x DMM).

(L)DI peridotites can be modeled by initial melting in the garnet field, followed by spinel field melting; however, LREE and FME enrichments cannot be replicated. In order to explain the enriched pyroxene compositions and truly reveal the origins of these peridotites, we further explore batch vs. fractional melting, employ an EMORB (or a DMM+EMORB mixture) source, and apply appropriate tectonic sensitive trace-element ratios. Our data and melt extraction models suggest that the (L)DI peridotites fit better within the framework of an arc environment (e.g., a back arc or a Cayman Trough-type trans-tensional setting), as opposed to a rifted margin, and that both DI and LDI are fault-bounded blocks in a post-Cambrian structural complex.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411269

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